1. Field of the Invention
The present invention relates to a grommet shackle for heavy duty fittings for lifting applications which are often subjected to forces of several hundred tons or more. In particular, the present invention is directed to a grommet shackle for heavy duty lifting applications which will, maximize the efficiency of the cable or rope attached thereto while minimizing the weight of the shackle itself.
2. Prior Art
Various designs for lifting shackles have been employed in the past. One example of an existing shackle is shown in Assignee's U.S. Pat. No. 4,079,584 issued Mar. 21, 1978 titled Heavy Duty Shackle.
In a typical shackle design, a pair of parallel ears or legs extend from a U-shaped bow. The ears or legs each contain an opening therethrough. The openings are aligned to receive a clevis pin or bolt. In large shackles for heavy duty applications, such as for use with marine anchors, the shackles may be rated for use at one hundred or even one hundred and twenty-five tons of force. These shackles themselves typically weigh over one hundred pounds and will often be moved with lifting equipment.
It would, therefore, be advantageous to minimize the weight of the shackle while maximizing the load capacity of the shackle.
Heavy duty shackles are employed with a variety of lifting equipment and fittings. Examples of fittings include a closed or endless loop wire rope sling with a swage sleeve and a wire rope sling with a standard eye terminator. Various types of wire rope configurations are utilized. In one known configuration, a central unit rope is surrounded by a plurality of helically wound outer ropes to form a wire rope or cable grommet. In another known configuration, groups of wire rope strands are themselves braided together to form a braided grommet. When a wire rope, such as used in a sling, is used with a shackle, the wire rope extends through the jaw of the shackle and engages the inner surface of the bow of the shackle.
When a wire rope or cable is bent over a shackle bow, hook or other lift point, the working load limit of the wire rope is reduced by a bending factor. The bending factor (EB) can be calculated according to the formula:
      E    B    ≡      1    -          0.5                        D          /          d                    
The diameter D represents the diameter of the bow around which the wire rope or cable is bent. The diameter d represents either diameter of the entire wire rope or cable grommet or, alternatively, represents the diameter of the individual wire strands. The D/d ratio is the ratio of the diameter around which the sling is bent divided by the diameter of the wire rope or cable.
It will be appreciated that the wire rope or cable, thus, has an efficiency related to various D/d ratios.
Accordingly, it is a principal object and purpose of the present invention to provide a heavy duty shackle which will maximize the lifting capacity while minimizing the weight of the shackle itself.
It is a further object and purpose of the present invention to provide a heavy duty shackle with increased load bearing surface.
It is a further object and purpose of the present invention to maximize the efficiency of the lifting equipment including any cable or rope by maximizing the diameter of the bow of the shackle.